1. Field of the Invention
The invention relates to the precise control of operation of a dynamometer and, more particularly, to a method and apparatus for controlling the driving (or motoring) or absorption load of a dynamometer to a targeted control level with great accuracy.
2. Description of the Prior Art
FIG. 1 is an illustrative block diagram of a conventional dynamometer load control apparatus which comprises a mechanical section including a dynamometer 1 having a rotary shaft 2 carrying at its one end an object 4 under test and at the other end a flywheel 3 for compensating the inertia term of the test object 4, a detecting section including a speed detector 5 for detecting the speed of the dynamometer 1, a torque detector 6 for detecting the load of the dynamometer 1, a current detector 7 for detecting the current flowing through the armature, and a voltage detector 8 for detecting the armature voltage, and a control section including a current adjustor 11 for adjusting the field current flowing through the field winding 10 of the dynamometer 1, an armature voltage adjustor 15, and control amplifiers 12 to 14 for providing a control output to the armature voltage adjustor 15.
The flywheel 3 may be removed if the control section is arranged so as to additionally control the inertia term of the test object 4. Although the present invention will be described in connection with the chassis dynamometer system, it is to be understood that the test object 4 may be set in an engine dynamometer fashion where an engine is directly set on the rotary shaft 2 or in a chassis dynamometer fashion where a vehicle is set on a roller coupled to the rotary shaft 2. Since the armature voltage adjustor 15 serves to control the armature voltage whether it is of the Ward Leonard type using a dynamo or of the thyristor type using a thyristor, it will be hereinafter referred merely to as a voltage adjustor. Additionally, since the field current adjustor 11 serves to adjust the field current flowing through the field winding 10 of the dynamometer 1 and thus the magnetic flux density of the dynamometer 1, it will be hereinafter referred merely to as a magnetic flux adjustor.
With such a conventional control system, the load of the dynamometer 1 is controlled as follows: The speed N of the dynamometer 1 is detected by the speed detector 5 and is introduced into the magnetic flux adjustor 11 which adjusts the field current I.sub.f flowing through the field winding 10 in accordance with the detected speed N such that the voltage V.sub.d (the product of the magnetic flux density .phi. and the speed N of the dynamometer) induced with rotation of the armature across the dynamometer 1 can be maintained substantially constant in order that the armature current I can be determined as a function of the control voltage of the voltage adjustor 15. On the other hand, the first control amplifier 12 receives a targeted control load F.sub.s and a load F detected by and delivered from the torque detector 6 and calculates the deviation between these two values and converts it into current form as a targeted control current I.sub.s. The second control amplifier 13 receives the targeted control current I.sub.s delivered from the first control amplifier 12 and also an armature current I detected by and delivered from the current detector 7 and calculates the deviation between these two values and converts it into voltage form as a targeted control voltage V.sub.s. The third control amplifier 14 receives the targeted control voltage V.sub.s delivered from the second control amplifier 13 and also a voltage V detected by and delivered from the voltage detector 8 and calculates the deviation between these two values to control the voltage level of the voltage adjustor 15. The armature current I provided under this control is given by EQU I=(V.sub.d -V.sub.g)/R (1)
where V.sub.d is the voltage induced by the dynamometer 1, V.sub.g is the control voltage of the voltage adjustor 15, and R is the total resistance of the armature circuit. The load generated in the dynamometer 1 with this armature current I is expressed by EQU F=K.phi.I (2)
where K is a constant determined by the kind of the dynamometer.
As can be seen from Equation (1), the direction of flow of the armature current I is dependent upon the relationship in magnitude between V.sub.d and V.sub.g. For example, the armature current I will flow in the direction indicated by the solid line of FIG. 1 and an absorption load is generated in the dynamometer 1 if V.sub.d &gt; V.sub.g, whereas it will flow in the reverse direction as indicated by the broken line of FIG. 1 and a driving load is generated in the dynamometer 1 if V.sub.d &lt; V.sub.g.
In such a conventional control system wherein the load F detected by the torque detector 6 is utilized as a feedback value for controlling the load of the dynamometer 1 as heretofore stated, the load F is required to be detected with great accuracy in order to provide precise dynamometer load control. However, the load F is normally obtained by detection of the reaction force of the rocking portion of the dynamometer 1. Thus, the load F cannot be detected in the range where it changes between the driving and absorption load states and also cannot be accurately detected in the small-load range due to the characteristic inherent in the torque detector.
In order to eliminate these difficulties, it has been proposed in an effort to calculate the load from the armature current I and the magnetic flux density .phi. in accordance with Equation (2). Unfortunately, it has been known for such an effort to be ineffective. Any attempt to calculate the magnetic flux density .phi. assuming that it is proportional to the field current I.sub.f will cause a great error and also any attempt to calculate the magetic flux density .phi. from measurable values such as the armature current I, its differentiated value, the terminal voltage, and the dynamometer speed will result in a value insufficient in accuracy.